How long a couple decides to wait in between having one child and the next could influence the second child’s risk of developing autism, a new study finds. Researchers from Columbia University have discovered the ideal window of time falls between two and five years.

The research team is quick to point out the results probably act more as a proxy for other environmental factors rather than make any claims themselves. So, while it’s not likely the case specific interpregnancy intervals — in this case, fewer than 12 months and longer than five years — have a direct impact on autism risk, these lengths of time could clue scientists into what’s going on in the background.

“The importance of this finding lies in the clues that it can provide in terms of understanding how the prenatal environment is related to outcomes after birth,” Dr. Keely Cheslack-Postava, lead researcher and psychiatric epidemiology training fellow at Columbia University, said in a press release.

Prior research has already determined autism and autism spectrum disorders (ASD) develop before the fetus’s brain is fully formed. What still eludes scientists is what factors in the environment, from air pollution, to genetics, to the mother’s diet, play the biggest role. These concerns are of particular importance as ASD continues to increase in prevalence. In the year 2000, one in 150 children was diagnosed. By 2010, the rate had risen to one in 68.

Cheslack-Postava and her team relied on a large data set — some 7,371 children born between 1987 and 2005 in Finland — to build their study. Roughly a third of the kids had been diagnosed with autism. Birth records from several national registries were used to compare the spacing of pregnancies among children who either did or did not have autism.

The results showed kids born less than a year after their siblings were one and a half times more likely to be diagnosed with autism. On the other end, kids born more than five years after their next-oldest sibling were 30 percent more likely, and outside the five-year threshold the difference in risk jumped to 40 percent.

"This study provides further evidence that environmental factors occurring during or near the prenatal period play a role in autism, a serious and disabling condition that afflicts millions of individuals and that is increasing in prevalence,” said senior author Dr. Alan Brown, a professor of psychiatry and epidemiology at Columbia.

In their research, the team accounted for several factors that have typically been thought to play a role in determining autism risk. These included parents' age, how many children they have, and their history of psychiatric disorders.

For women looking to reduce the chance their child will develop autism, there is a body of science that argues omega-6 fatty acids may help reduce risks. However, these healthy fats are only healthy up to a certain dosage, experts argue. Past the threshold, the omegas lose their benefit for the growing fetus. Women who consumed omega-6 acid were 34 percent less likely to birth a child with autism. Meanwhile, those who consumed low levels of omega-3 fatty acids were 53 percent more likely.

As for future research, Brown credits the latest study as setting a reliable precedence with its addition of a time component — though, there may be other benefits, too, he says. “This work also exemplifies the importance of large samples of pregnancies with data acquired during pregnancy and their linkage to comprehensive, national databases of reproductive factors and psychiatric diagnoses.”

Source: Cheslack-Postava K, Suominen A, Jokiranta E, et al. Increased Risk of Autism Spectrum Disorders at Short and Long Interpregnancy Intervals in Finland. Journal of the American Academy of Child & Adolescent Psychiatry. 2014.